A boiling water nuclear reactor (BWR) includes a pressure vessel containing a nuclear reactor core and water which is boiled thereby for generating steam for producing power such as by driving a turbine-generator for producing electrical power. The pressure vessel typically includes a cylindrical shell having a removable head at the top, and an integral head at the bottom thereof. The top head typically includes an arcuate dome having an integral mounting flange which mates with a complementary supporting flange on the shell. A plurality of circumferentially spaced apart bolts extend through the head flange into the shell supporting flange for bolting the upper head to the shell to form a pressure vessel capable of withstanding the relatively high pressures generated within the pressure vessel during operation of the reactor core.
The upper head typically includes an integral nozzle at the top center thereof which is conventionally joined to a vent line, or a spray line, or both. The vent line communicates with the head nozzle for releasing or venting non-condensable gases such as nitrogen before startup of the reactor core, for example. And, the spray line communicates with the nozzle for injecting or spraying water therein for cooling purposes, for example. In both configurations, the vent and spray lines must be disconnected from the upper head prior to removing the upper head from the shell during a maintenance operation. This increases the complexity and time for accomplishing routine maintenance and increases the duration of radiation exposure to maintenance personnel.